Generally, direct casting methods are used to cast molten aluminum into sheets. According to such methods, molten aluminum is first fed to a reservoir. The molten aluminum goes from the reservoir to a nozzle section. The nozzle of this section is a chilling nozzle. The molten aluminum solidifies as it issues from chilling nozzle.
A pair of water-cooled roller caster shells are disposed adjacent the end of the chilling nozzle. These shells, which rotate in opposite directions, receive the solidifying, yet malleable, aluminum in their nip as it issues from a chilling nozzle. The roller caster shells hot roll the solidifying aluminum into sheets. Depending on the size and separation of the shells, aluminum sheets of various thicknesses and widths are produced.
Since the shells are water-cooled, they further cool the aluminum as it passes through the nip. As the aluminum sheet leaves the nip, it is cooled to point at which it will maintain its shape or form.
During each thermal cycle in the casting process, roller caster shells are subject to extreme heat stresses. These heat stresses cause heat checking or cracking ("heat cracking") in the shells. If the cracking goes unattended, the heat stresses in subsequent thermal cycles will propagate the cracking. The cracks that are formed cause marks in the aluminum sheet, which is undesirable, and the propagating of cracks eventually renders the shell unusable. The speed of crack propagation in a given shell determines the service life of a given shell.
Manufacturers of aluminum sheet have always wanted roller caster shells with service lives as long as possible for both practical and economic reasons. Prior art roller caster shells for continuously roll casting aluminum into sheets have various chemistries. The various prior art chemistries stress the criticalness of the weight percentages of one or more constituent elements. Meeting these specific weight percentages for these elements is the basis for the alleged increased service life of such prior art shells.
To increase the service life of roller caster shells, the apparatus upon which the shells are mounted is periodically shut down to machine the outer surface of the shells. Remachining removes the cracks before they have a chance to propagate in either length and depth. Each remachining of the shells reduces the thickness of the shells usually between 5% to 15%. However, the remachining process greatly extends the service life of the shells compared to what it would be without remachining. Normally, each shell can be remachined five (5) times before it has to be scrapped.
Shell service life can be measured in two ways: (1) the number of hours of operation of the shells before replacement; or (2) the number of pounds of aluminum processed before replacement. At best, prior art shells can allegedly process approximately 10 million pounds of aluminum, which equates to approximately 5000 hours of operation, before being scrapped.
The present invention provides a ferritic alloy steel for roller caster shells which has high resistance to heat cracking. The chemistry for the roller caster shell of the present invention has weight percentage ranges different from those of prior roller caster shell steels--in fact, the ranges for some of the elements are contrary to conventional teachings with respect to ferritic alloy steels for roller caster shells with long service lives.